Within transmitting and receiving arrangements in mobile stations, modulators and demodulators are used which modulate the data to be transmitted onto a radio-frequency carrier oscillation, and demodulate them for reception, in accordance with standardized modulation and demodulation methods. One of these mobile radio standards is the TD-SCDMA (Time Division Synchronous Code Division Multiple Access) Standard.
For simplicity, the TD-SCDMA method can be described as a combination of the GSM (Global System for Mobile Communications) method and the CDMA (Code Division Multiple Access) method. The transmission time is subdivided into time slots in the TD-SCDMA method in precisely the same way as in the GSM method. Furthermore, the data to be transmitted from a transmitter is coded using orthogonal code words in the sense of a code-division multiplexing method, with the TD-SCDMA method being characterized by a chip rate of 1.28 MHz. The orthogonal spread coding makes it possible for a number of users to transmit and to receive on the same frequency at the same time.
In the case of the TD-SCDMA method, the transmission and reception of signals by a subscriber do not take place at the same time, and, instead of this, a subscriber transmits and receives during different time slots. This so-called half-duplex method has the advantage that the transmitting and receiving arrangements do not need to be designed to be as complex. A further advantage of the TD-SCDMA method is that the data rates need not be symmetrical for the two transmission directions.
Further information relating to the TD-SCDMA Standard can be found in the TD-SCDMA specification “China Wireless Telecommunication Standard (CWTS); 3G digital cellular telecommunications system; TD-SCDMA System for Mobile (TSM); Radio transmission and reception (Release 3)”, CWTS TSM 05.05 V.3.0.0 (2002-08).
Generally, the modulators and demodulators in transmitting and receiving arrangements in mobile stations comprise a baseband component and a radio-frequency component. During transmission, the baseband component uses digital signal processing to create a generally complex signal, which complies with the standard, from the data to be transmitted, wherein complex signal is shifted to radio frequency by the radio-frequency component and is transmitted via the antenna as a real signal after suitable amplification. During reception, the received payload data is demodulated in a corresponding form by the radio-frequency component to form a complex signal. The received data is then processed further in the baseband component.
Owing to the different physical requirements for the baseband and radio-frequency components, these functional units are generally produced in separate integrated circuits using different production technologies. In the transmission direction, the modulated baseband signal must be passed in a suitable form to the radio-frequency component. In the reception direction, the demodulated radio-frequency signal must be passed in a suitable form to the baseband component. A suitable interface must be provided between the baseband component and the radio-frequency component for this purpose.
In this case, the transmission between the baseband component and the radio-frequency component is normally in analogue form. However, this has the disadvantage that analogue components are also required both in the baseband component and in the radio-frequency component, which is disadvantageous, for example, bearing in mind the progress in digital semiconductor technologies.
In conventional mobile stations that are based on the TD-SCDMA Standard, a complex analogue signal, represented by a real and an imaginary part, is produced in a hybrid baseband module (mixed signal module) which has both digital and analogue circuit components, and, in order to achieve as high a signal transmission quality as possible, is transmitted to the radio-frequency assembly in the form of differential analogue signals, and is received by the radio-frequency assembly. Those circuit parts that are required for production and transmission of the differential signals in particular occupy a significant proportion of the chip area and are not required for the actual signal processing. As a result of the analogue transmission, the interface is also sensitive to radiated interference and noise, and requires four analogue line connections just for the transmission of the payload information.
In addition to the payload information, configuration information and synchronization information is transmitted via various analogue and digital control lines and via a serial bus, thus resulting in further lines.
The German Laid-Open Specification DE 100 35 116 A1 describes a conventional analogue radio-frequency interface for dual-standard baseband chips in mobile radios.